1. Field of the invention
The present invention relates to a method for preparing polyaniline films, in particular with good charge transport properties in semiconducting and/or conducting films, as well as to the polyaniline (PANI) obtained, and to uses thereof.
2. Description of the Related Art
The creation of novel materials and novel technologies for electronics or optoelectronics in particular is based on researching conjugated polymers with conducting or semiconducting properties. Organic semiconductors are becoming an alternative to inorganic semiconductors for the development of simple electronics and for the production of such materials in large quantities and cheaply.
For certain applications, one of the major problems is to obtain materials with good uni-directional charge transport properties. As an Example, the development of transistors requires an alignment of π-π interactions parallel to the surface of the semiconductor film, and the results obtained up to now have not been sufficient in this respect.
This is the case with polyaniline, which is one of the best candidates for the various envisaged applications, primarily due to the low cost of the starting monomer, and due to the fact that PANIs are easy to obtain, for example by oxidizing aniline in aqueous solution, and due to the fact that the properties of the PANIs obtained are advantageous: they have thermal and chemical stability, and electronic, optical and optoelectronic properties.
Polyaniline is obtained in the following forms:

The emeraldine base and the last two forms are insulators.
Usually, the organization of the polyanilines is controlled by choosing treatments following polymerization. At the end of the standard polymerization of aniline, semi-crystalline polyaniline (emeraldine salt) is obtained in the form of an insoluble and infusible powder. In order to obtain orientated films, various treatments carried out after polymerization, or “post-polymerization treatments” or “post-treatments” have been envisaged and several of those treatments are indispensable: neutralization, washing, drying, dissolution in appropriate organic solvents or post-protonation with organic sulphone-containing organic acids, and drying on a support followed by stretching of the films obtained. Polymer fibres may also be obtained by extrusion, stretching, etc.
Until now, two principal approaches for obtaining crystalline films based on π-conjugated oligomers or polymers have been envisaged. The first consists of evaporating π-π conjugated polymers under vacuum. The second is based on the use of solutions of soluble derivatives.
It is thus known to dissolve emeraldine base in N-methylpyrrolidone (NMP) and to orientate the film by stretching it, the NMP acting as a plasticizer. It is also known that, when protonated by camphorsulphonic acid (ACS), polyaniline is partially soluble in meta-cresol, which means that films can be obtained by evaporation of meta-cresol. However, all of the pathways employed until now to obtain such associated polyaniline chain forms necessitates and brings about the destruction of its native semi-crystalline organization.
The degree of crystallinity of the films obtained does not usually exceed 40% in general.
In order to illustrate this prior polymerization technique, usually carried out in the presence of Cl−, U.S. Pat. No. 5,276,112 may be cited; it describes a method for manufacturing PANIs by polymerization using ammonium persulphate, in which a first solution comprising aniline, a protonic acid, a salt and a second solution comprising an acid, a salt and ammonium persulphate as a polymerization agent are mixed, then the PAM precipitates out and it is washed. The protonic acid is selected from HCl, HBr, HI, HClO4, H2SO4, H3PO3 and CH3COOH. The PANI may be dissolved in various solvents to form it into a film or a fibre. In order to obtain the film, the solvent is evaporated off.
Further, in order to overcome the problem linked to the presence of chlorine in the PANIs obtained and/or to the use of LiCl or Cl− during polymerization, US-2007/0249803 describes the use of formic acid in the polymerization of PANIs with a high molecular weight which are free of chlorine. The polymerization of aniline is carried out in the absence of chlorine and in the absence of salts intended to lower the freezing point using a non-chlorinated acid at concentrations where it prevents the medium from freezing. More precisely, it describes the polymerization of aniline carried out at −25° C. in the presence of ammonium persulphate, in the absence of salt capable of lowering the freezing point of the polymerization medium, in the presence of formic acid in quantities of approximately 10 or even 20 times higher than that of the aniline used, followed by deprotonation carried out using an ammonium hydroxide solution, and washing of the polyaniline powder is carried out with water then with 2-propanol.
There is a need for films of highly crystalline and self-orientated PANI that can be used without post-treatment other than drying, or without a residue of processing agent or film-forming agent. The Applicant has demonstrated that by making a particular selection of the conditions for carrying out the method for oxidative polymerization in an aqueous acid phase, films can be obtained directly after synthesis which have a strong orientation of π-π interactions parallel to the surface of the film.